Should this be an automatic solution, i.e. you specify the threshold level, or do you want to specify the horizontal position?
– JakeJul 16 '13 at 22:12

I know the positions beforehand, I just need to draw the markers.
– joaocandreJul 16 '13 at 22:29

Do you want to be able to specify the heights of the lines? If yes, how do you want to specify them? Same thing for the widths of the bars, do you want to specify them in terms of the axis units, or in paper units?
– JakeJul 16 '13 at 22:36

The problem with axis units is that I am using a logaritmic scale, so I'd rather specify the width of the bar in paper units. The height is fine either way.
– joaocandreJul 17 '13 at 1:11